Temperature compensated magnetic transducer



March 7, 1967 G. R. CURTIS ET AL 3,308,412

TEMPERATURE COMPENSATED MAGNETIC TRANSDUCER Original Filed April 19.1960 United States Patent illice 3,308,412 TEMPERATURE COMPENSATEDMAGNETIC TRANSDUCER Gerald R. Curtis, Duarte, James C. Kyle, Glendora,and Glen Robinson, Pasadena, Calif., assignors, by mesne assignments, toPhysical Sciences Corporation, a corporation of California Originalapplication Apr. 19, 1960, Ser. No. 23,267, now Patent No. 3,183,126,dated May 11, 1965. Divided and this application June 13, 1963, Ser. No.300,950

8 Claims. (Cl. 336-136) This is a division application Serial No.23,267, liled April 19, 1960, and now Patent No. 3,183,126.

This invention relates to magnetic transducers and to methods of makingsuch transducers and, more particularly, this invention relates to hightemperature variable permeance transducers and to a method of makingsuch transducers.

There are applicati-ons in which it is desirable to accurately measure aphysical input such as pressure in a high temperature environment. Forexample, in a nuclear reactor, measuring instruments and transducers arerequired for operation in temperature environments up to 1,000 degreesFahrenheit and in the presence of substantial radiation. In suchenvironments, conventional instruments .and transducers bec-omeinaccurate and often inoperable.

One conventional transducer utilized t'o transform a positional input toan electrical signal is a Variable Permeance Transducer. The inputmovement may be Iresponsive, illustrfatively, to the -action of adiaphram of a pressure sensitive capsule. The Variable PermeanceTransducer generally includes a rst. and a second wound on anon-magnetic tube which encloses a moving element in the form of a coremade of high permeability magnetic material. With the core centered inthe tube, the permeance of the two windings is the same. As the core ismoved toward either end, the permeance of o-ne winding is increased, andit is decreased in the other winding. The

result is that an output voltage is produced which is pro-V .portionalto the displacement of the core.

The core is, of course, made of ferromagnetic materials and, therefore,is an alloy composed essentially of one or a combination of threeelements, iron, cobalt and nickel which are the only three elementsferromagnetic at room temperature. The manufacture of a ferromagneticalloy is generally a detailed process designed to enhance the desirablemagnetic characteristic of the alloy. The magnetic properties of asingle crystal of iron or nickel depend upon the crystallographicdirection, in which they are measured. The crystal of grain orientationis effective in enhancing certain magnetic characteristics, and twomethods are generally utilized to -orient the crystals; cold rolling,and annealing, both in the presence of a magnetic field.

A core ma-de of a conventional ferromagnetic alloy must be handledcarefully because magnetic or thermal shocks or deformations degrade themagnetic properties of these special alloys. Even without shock, avariation of temperature changes the magnetic `characteristic of thematerial. Conventional core materials such as Deltamax Orthonal, 48Alloy, Hypernik-V, Hy-Mu 80, 4-79 Mo- Permalloy `and Sfuper-Permalloy(all U.S. trademarks) change as much as 10 to l5 percent when heatedeven gradually to 1,000 degrees Fahrenheit. None of the conventionalco-re materials are, accordingly, suitable for use to provide for anaccurate magnetic transducer for use in high temperature environments.Moreover, all these conventional core materials are not high temperaturema- 3,308,412 Patented Mar'. 7, 1967 terials and, accordingly, theyoXidize at high temperatures and tend to fall apart. The oxidationeiectively seperates the crystals of the ferromagnetic material.Further, thermal shocks or rapid changes in temperature speed thedistintegration of the core m-aterials.

Moreover, conventional magnetic materials are all affected by theradioactivity present in nuclear reactor so that their magneticcharacteristics change substantially responsive thereto. Morespecifically, with changes of temperature, and in the presence lofradioactivity, the curie point at which the material loses itsferromagnetic properties decreases, the saturation flux density andrectangularity of the hysteresis loop changes; and the coercive forcealso is changed. These changes are not readily predictable and,therefore, cannot readily be compensated for by components external tothe transducer. Because of these various factors, suitable transducersfor use in nuclear reactors have not heretofore been provided.l

In a specific illustrative embodiment of Ithis invention, a variablepermeance transducer is provided including .two matched impedances inthe form of windings of a suitable material such as aluminum. Aluminumin effectively transparent to radioactive particles. The aluminum iscoated with a particular ceramic material which maintains its insulatingproperties at temperatures in the range of 1,000 degrees Fahrenheit andin the radioactive environment in the reactor. The inductive balance |ofthe transducer is varied by means of the movement of a slug or core madeessentially of a high temperature chromium-iron alloy. Features of thisinvention relate to a method of removing stresses from the core materialand for magnetically stablizing the core material so that neither thestresses nor the magnetic characteristics of the material effectivelychange with substantial variations oftemperature. The stresses areremoved and the stability of the 'material is improved by repeatedlythermoshocking the material to randomly Iorient the alloy crystals andVto lock them in the random orientation. In one embodiment of theinvention, a ferromagnetic stainless steel material is subjected to anumber of different thermal shocks to stabilize the material bothdimensionally and magnetically. The thermoshocks increase the modulus ofelasticity of the material and increase its hardness. The permeabilityof the material is somewhat less than that of the conventional corematerials but, due to the tact that the Output voltage is derived by themagnetic unfbalance of the two windings, he sensitivity of thetransducer is maintained.

Other features of this invention pertain to the provision of means forcompensa-ting for any small changes of the magnetic characteristics ofthe core at the elevated temperatures. The compensating means is in theform of the resistivity of the aluminum windings which increases withtemperature as does the magnetic ilux developed by the core. The changeof magnetic characteristics of .the

core of this invention is quite Small, illustratively 0.001

percent per degree Fahrenheit compared to 10 percent per degreeFahrenheit for the -conventional core materials.

Further advantages and features of this invention will become apparentupon consideration of the foilowing description when read in conjunctionwith the drawing i wherein:

FIGURE 1 is a pictorial view of the variable permeance transducer ofthis invention;

Vsection of the order of only 30 barns.

FIGURE 4 is an electrical representation of the variable permeancetransducer of this invention.

Referring to FIGURES 1 through 4, the variable permeance transducer 10is cylindrically shaped, and includes a tube or shell 11 made of anon-magnetic material. The material may illustratively be a non-magneticsteel. The tube 11 supports two windings 20 and 21 made of a -materialsuch as aluminum which is effectively a window for radioactiveparticles. Copper, which is conventionally utilized for transducerwindings, becomes somewhat yradioactive and changes its characteristicsin the presence of the radiation. The aluminum wire of winding 20,coated with an insulating material, is wound on the tube 11 between twobailles 12 and 13 of the tube 11, and the alumi num winding 21, alsocoated with an insulating material, is wound on the tube 11 between twobattles 13 and 14. The baffles 12, 13 and 14 are shown particularly inFIG- URE 3 together with two end battles 19 and 16 positioned at therespective ends of the tube 11. The baffles 12, 13 and 14 hold thewindings 20 and 21 in place, and each includes a number of peripheralslots 17 for connecting leads between the windings 20 and 21 and forconnections to external components, not shown. The windings 20 and 21may be made of aluminum conductors l0 mils in diameter and approximategage of 30.

As illustrated in FIGURE 4, the two windings 20 land 21 have one commonterminal 30. The three terminals 30, including the common terminal, areconnected through circular openings 19 in the end baille 15 of the tube11. The terminals, or pins, 30 are supported by small ceramic insulatorbushings, or beads, 34 in the openings 19 an-d maybe made ofnon-magnetic stainless steel such as AISI 304. The tube 11 is coatedwith an inorganic ceramic material to fully insultate it from thewindings 20 and 21. The ceramic material coating may be similar to thematerial of the ceramic bushings 34 and the insulator coating on thewindings 20 and 21.

As indicated above, the aluminum windings 20' and 21 are also coatedwith a ceramic material which may be similar to that of the beads 34 andthe coating of the tube 11. A suitable method for coating aluminum witha ceramic material which maintains its insulating properties attemperatures in a range of 1,000 degrees Fahrenheit is disclosed in thecopending patent application Serial No. 847,081 filed on October 19,1959, by John A. Earl, and

now abandoned. As described in the copending patentapplication, thecoating may consist of a mixture' by weight of lead oxide from 70 to 76percent silicon dioxide, 'from l to 1.4 percent bismuth trioxide, from 7to 14 percent, and from 4 to 6 percent of any one of barium oxide, lannthium trioxide, magnesium oxide, calcium oxide and zinc oxide. Thecoating is substantially unaffected by nuclear flux because it has a lowthermoneutron capture cross Moreover, as indicated above, the coatingmaintains its electrical insulation at temperatures on the order of1,000 degrees Fahrenheit. The various ingredients of the mixture arethoroughly mixed and then smelted until homogenized at a tempera-ture ofapproximately 2,100 degrees Fahrenheit. After being homogenized, themixture is quenched in water and then ground through a line mesh screen.The mixture then is coated on the aluminum and tired to a suitablefiring temperature between 1,000 degrees Fahrenheit and 1,200 degreesFahrenheit to cure the coating. The resistivity of the coating at roomtemperature is on the order of 1 l014 ohms, and the resistivity `at1,000 degrees Fahrenheit is on the order of 4X10lohms. As indicatedabove, the ceramic coating of the tube 11 and the ceramic beads 34 maybe made of similar material. The windings 20 and 21 are enclosed by acylindrical tube 32 which may be made of steel such as the non-magneticstainless steel having the AISI designation of 304. The tube 11 may bemade of a similar material. The tube 32 may also be coated with theceramic insulating material.

The tube 11 encloses a slug or core 25 which is made of magneticmaterial and which is movable longitudinally in the tube 11. Two leads26 and 27 are axed respectively to the core 25 and extend through theend baies 16 and 19 of the tube 11. The inductance presented by each ofthe two windings 20 and 21 and the coupling therebetween is determinedby the longitudinal position of the core 25. With the core 25 centered,the inductance of the two windings 2t) and 21 is identical so that anull or minimum signal is provided at the common terminal 30.

As described above in the introduction, the magnetic characteristics ofthe core 25 must remain substantially constant throughout a temperaturerange up to 1,000 degrees Fahrenheit and in the presence of radiation.In order to magnetically sta-blize the material of the core 25,thevfarious crystalline stresses of the core 25 are removed. Thematerial is so stabilized as to have a variation of magneticcharacteristics only 0.001 percent per degree Fahrenheit throughout atemperature range from minus 320 degrees Fahrenheit to 1,000 degreesFahrenheit. Moreover, the curie `point temperature of the material isincreased and the ymaterial is not effectively susceptible to theradioactive environment.

These characteristics are achieved by a thermoshocking process utilizinga ferromagnetic high temperature material. The material illustrativelymay be a magnetic stainless steel material or alloy consisting mainly ofchromiurn and iron. One particular illustrative embodiment of thematerial includes by weight, 12 to 14 percent of chromium, 0.5 percentnickel, 1.25 percent manganese, 1 percent silicon, 0.15 percent carbonand the rest of iron. Such a stainless steel is conventionallydesignated AISI 416. The stainless steel is readily machinable and mayalso include a minimum of 0.07 percent by weight of phosphorous, sulfuror selenium; or a maximum of 0.06 percent by weight of zirconium ormolybdenum. Any one of the AISI 400 series of stainless steels may bemagnetically stabilized. The particular stainless steel 416 is selectedbecause it is also readily machinable. The AISI 300 are not magnetic andcannot be hea-t treated. The magnetic characteristics of the 400 seriesstainless steels are generally deleterious in conventional applicationsof such steels.

One specific illustrative process for magnetically stabilizing thematerial to form a suitable core 25 for the variable permeancetransducer 10 includes the following steps:

(l) The stainless steel material is heated to 1,550 degrees Fahrenheitand maintained at that elevated temp erature for 8 to 10 hours;

(2) The heated material is then cooled to room temperature;

(3) The material is dropped into liquid nitrogen at -320 to 325 degreesFahrenheit and maintained in the liquid nitrogen for aproximately 15minutes;

(4) The cooled material is then warmed back to room temperature;

(5) The material is then heated to approximately 1,100 degreesFahrenheit for 1 hour;

(6) The heated material is again returned to room temperature;

(7) The material is then dropped back into liquid nitrogen atapproximately 320 degrees Fahrenheit for another 15 minute interval;

(8) The material is then returned to room temperature;

(9) The material is again heated to 1,100 degrees Fahr enheit for onehour;

(10) The material is returned to room temperature;

(1l) Again, the material is thermally shocked to 320 degrees Fahrenheitin the liquid nitrogen for an interval of 15 minutes;

(12) The cooled material is then returned to room temperature;

(13) The substantially stabilized material is then cut and turned downto 1t-inch diameter to remove surf-ace of the material;

(14) The cut material is then again heated to 1,100 degrees Fahrenheitfor 1 hour to remove stresses in the material due to the machiningoperation;

(15) The heated material is returned to room temperature;

(16) The material is then thermally shocked to 320 degrees Fahrenheit inthe liquid nitrogen for 15 minutes;

(17) The material then returned to room temperature;

(18) The piece of material is then turned down to 0.125 inch` which isjust slightly larger than that required for the particular magneticcore;

(19) A cylindrical hole 0.0509 inch in diameter is drilled through thecore along its longitudinal axis;

(20) The drilled material is then heated up again to 1,100 degreesFahrenheit for 15 minutes, returned to room temperature, and droppedint-o the liquid nitrogen to remove the stresses in the material due todrilling;

(21) The material is then returned to room temperature;

(22) Finally, the piece of magnetic material is centerless ground tocorrect its external diameter to 0.116 inch i-0.003 inch and to a lengthof 1.250 inches $00005 inch. The last step is essentially a surfacecleaning operation.

The particular dimensions are, of course, merely illustrative and aregiven to illustrate the feature of machining the material to approximatesize then thermoshocking the material again to remove machine stresses.Any of the conventional core materials would oxidize at the elevatedtemperatures and would fall apart during the repeated thermoshocks. Theparticular selected material for the core is a high temperature materialwhich does not oxidize at the elevated temperatures. Moreover, thematerial is readily machinable so that only small stresses areintroduced to the material by the machining operation.

The initial thermoshock cycles stabilize the dimensions of the corematerial. The machining operations are provided after the dimensionshave been stabilized to accurately provide the desired core dimensions.

A core 25 produced in accordance with this method is very stable andprovides for minute changes of magnetic characteristics for temperaturesup to 1,000 degrees Fahrenheit. Even the small change in magneticcharacteristics, however, is compensated for by an opposite small changein the resistance of the aluminum wire with changes of temperature. Theresistivity of the aluminum Wire increases with changes of temperatureto decrease the elfective flux density developed by the core. Theresistivity illustratively increases from approximately 20 ohms percircular mil foot to approximately 60 ohms per circular mil foot. Thecurie point of the core 25 is quite high, at approximately 1,580 degreesFahrenheit.

Although this inventio-n has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

What is claimed is:

1. An electromagnetic transducer having stable characteristics whenexposed to variable radioactive and uctuating high temperatureenvironments, the transducer comprising:

a tube made of non-magnetic material;

at least one winding formed from an elongated material coated with aboron-free ceramic disposed on the Vtube and having atemperature-resistance characteristic eifectively immune to variationsin radioactivity over an extended range of temperatures to approximately1000" F.;

a coating of boron-free ceramic on the peripheral surface of the tubeand having a temperature-resistance characteristic effectively immune tovariations in radioactivity over an extended range of temperatures toapproximately 1000 F.; and

disposed within the tube, a core having a temperaturepermeabilitycharacteristic counteracting the effect of the temperature-resistancecharacteristic of the winding and having substantially constant magneticcharacteristics through an extended range of temperatures toapproximately 1000 F. and having characteristics effectively immune tovariations in radioactivity.

2. An electromagnetic transducer having stable characteristics whenexposed to variable radioactive and fluctuating high temperatureenvironments, the transducer comprising:

a tube made of non-magnetic material;

at least one winding formed from aluminum coated with a boron-freeceramic disposed on the tube, and having a temperature-resistancecharacteristic effectively immune to variations in radioactivity over anextended range of temperatures to approximately 1000 F.;

a coating of the boron-free ceramic on the peripheral surface of thetube; and

disposed within the tube, a magneti-c core consisting mainly of chromiumand iron and having a temperature permeability characteristiccounteracting the effect of the temperature-resistance characteristic ofsaid Winding and having substantially constant magnetic characteristicseven when subjected to radioactivity and having substantially constantmagnetic characteristics over an extended range of temperatures betweenapproximately 320 F. and approximately 1000 F.

3. An electromagnetic transducer having stable characteristics whenexposed to variable radioactive and fluctuating high temperatureenvironments, the transducer comprising:

a tube made of non-magnetic material;

at least one winding formed from aluminum coated with a boron-freeceramic disposed on the tube and having a temperature-resistancecharacteristic effectively immune to variations in radioactivity over anextended range of temperatures to approximately 1000 F.;

a coating of boron-free ceramic on the peripheral surface of the tube;and

disposed within the tube, a core having a temperature- 'permeabilitycharacteristic counteracting the effect of the temperature-resistancecharacteristic of the winding, said core comprising a piece of magneticstainless steel including by weight 12 to 14 percent chromium, 0.5percent nickel, 1.25 percent manganese, l percent silicon, 0.15 percentcarbon, and the rest iron With characteristics of providingsubstantially a constant permeability when subjected to radioactivityand over an extended temperature range between approximately 320 F. andl000 F.

4. The electromagnetic transducer set forth in claim 1 wherein theboron-free ceramic consists of a mixture by weight of lead oxide fromabout 70 percent to 76 percent, silicon dioxide from about 10 percent to14 percent, bismuth trioxide from about 7 percent to 14 percent and fromabout 4 percent to 6 percent of a material selected from a groupconsisting of the oxides of barium, magnesium, calcium and zinc.

5. The electromagnetic transducer set forth in claim 1 wherein the coreconsists of 12 percent to 14 percent of chromium, 0.5 percent nickel,1.25 percent manganese, 1 percent silicon, 0.15 percent carbon and thevremainder of iron.

6. The electromagnetic transducer set forth in claim 2 wherein the coreincludes 12 percent to 14 percent of chromium -and approximately 83percent to 85 percent of 1ron.

7..The electromagnetic transducer set forth in claim 6 wherein theboron-free ceramic consists of a mixture by weight of lead oxide fromabout 70 percent to 76 percent, silicon dioxide from about 10 percent to14 percent, bismuth trioxide from about 7 percent to 14 pericen-t'andfrom about 4 percent to 6 percent of a material selected from a `group`consisting of the oxide of barium, magnesium, calcium and zinc.

8. The electromagnetic transducer set forth in claim 3 wherein theboron-free ceramic consists of a mixture by Weight of lead oxide fromabout 70 percent to 76 percent, silicon `dioxide from about 10 percentto 14 percent, bismuth trioxide from about 7 percent to 14 percent andfrom about 4 percent to 6 percent of a material selected from a groupconsisting of the oxides of barium, magnesium, calcium and zinc.

References Cited by the Examiner UNITED STATES PATENTS 3,052,576 9/1962Josso 148-135 X 3,060,353 10/1962 Shansky et al 336-213 X 3,089,0815/1963 Brosh 336-30 X 3,119,897 1/1964 Coper 174-110 OTHER REFERENCESRadiation Shielding: Price, Horton and Spinney 1957.

LEWIS H. MYERS, Primary Examiner. C. TORRES, T. KOZMA, AssistantExaminers.

Dedication 3,308,412.Gemld R. urtz's, Duarte, James Kyle, Glendora, andGlen Robinson, Pasadena, Calif. TEMPERATURE COMPENSATED MAGNETICTRANSDUCER. Patent dated Mar. 7, 1967. Dedication filed June 3, 1970, bythe assignee, Physical Sciences Cowpomtz'on. Hereby dedicates the entireterm of said atent to the Public.

[Oficial Gazette November 10, 1.970.?

1. AN ELECTROMAGENTIC TRANSDUCER HAVING STABLE CHARACTERISTICS WHENEXPOSED TO VARIABLE RADIOACTIVE AND FLUCTUATING HIGH TEMPERATUREENVIRONMENTS, THE TRANSDUCER COMPRISING: A TUBE MADE OF NON-MAGNETICMATERIAL: AT LEAST ONE WINDING FORMED FROM AN ELONGATED MATERIAL COATEDWITH A BORON-FREE CERAMIC DISPOSED ON THE TUBE AND HAVING ATEMPERATURE-RESISTANCE CHARACTERISTIC EFFECTIVELY IMMUNE TO VARIATIONSIN RADIOACTIVITY OVER AN EXTENDED RANGE OF TEMPERATURES TO APPROXIMATELY1000*F.; A COATING A BORON-FREE CERAMIC ON THE PERIPHERAL SURFACE OF THETUBE AND HAVING A TEMPERATURE-RESISTANCE CHARACTERISTIC EFFECTIVELYIMMUNE TO VARIATIONS IN RADIOACTIVITY OVER AN EXTENDED RANGE OFTEMPERATURES TO APPROXIMATELY 1000*F.; AND DISPOSED WITHIN THE TUBE, ACORE HAVING A TEMPERATUREPERMEABILITY CHARACTERISTIC COUNTERACTING THEEFFECT OF THE TEMPERATURE-RESISTANCE CHARACTERISTIC OF THE WINDING ANDHAVING SUBSTANTIALLY CONSTANT MAGNETIC CHARACTERISTICS THROUGH ANEXTENDED RANGE OF TEMPERATURES TO APPROXIMATELY 1000*F. AND HAVINGCHARACTERISTICS EFFECTIVELY IMMUNE TO VARIATIONS IN RADIOACTIVITY.